Electron microscopy offers the opportunity to study material architectures in 3D at high resolution. This technique may be used to observe and analyze a variety of materials and used in different fields of applications. Although this invention is often utilized in the observation and analysis of semiconductors (e.g., via, transistors, etc.), it should be understood that the present invention is not limited to semiconductors and may include other materials such as metals, catalysts, polymers, and biological structures, for example. One technique employed with electron microscopy for analyzing materials, for example, is called “slice and view.” This technique is typically performed with a system combining a focused ion beam (FIB) and a scanning electron microscope (SEM) such as the DualBeam® instruments commercially available from FEI Company, the assignee of the present invention.
In the slice and view technique, as illustrated by FIG. 1, a feature of interest in a sample material is located and measured by known methods and techniques. The FIB cuts and slices the sample with high precision to reveal its 3D internal structures or features. Typically, the FIB exposes a cross section, or face, perpendicular to the top of the surface of the sample material having the hidden feature to be viewed. To further assist with separating a slice from the substrate material on each side of the area of interest is removed. Because the SEM beam axis is typically at an acute angle relative to the FIB beam axis, a portion of the sample in front of the face is preferably removed so that the SEM beam can have access to image the face. After obtaining an image of the face by the SEM, another layer of substrate at the face may be removed using the FIB, revealing a new, deeper face and thus a deeper cross-section of the feature. Since only the portion of the feature at the very surface of the face is visible to the SEM, sequential repetition of cutting and imaging, or slicing and viewing, is performed until the run is complete. This process provides the data needed to reconstruct the sliced sample into a 3D representation of the feature. The 3D structure is then used to analyze the feature.
One problem that may occur while slicing away a layer of the substrate is redeposition. That is, particles of the remnant material may redeposit on the face of the feature to be imaged, preventing an unobstructed view of the face to be imaged causing an undesirable image resolution. This is best seen in FIG. 2, showing a standard slice and view method in which the FIB is normal to the top surface of the sample and the SEM is typically angled at about 52 degrees relative to the FIB axis. In this method, a vertical wall, or face 20, is exposed by removing material using the FIB to form a sloping trench 24. Once face 20 is exposed it is viewed for imaging by the SEM. A slice of material 26 may be removed using the FIB to expose a new face for imaging by the SEM. As the FIB performs the milling operation ablated material 28 from the area being milled and gallium from the ion beam may build up in front of and may be redeposited back onto face 20 altering or obscuring face 20. FIG. 3 illustrates the same problem in an angled slice and view method in which the SEM is normal to the top surface of the sample and the FIB is angled relative to both the SEM axis and the top surface of the sample. In this method, an angled face 30 is exposed by removing material using the FIB to form a trench 32. When face 30 is exposed it is viewed for imaging by the SEM. A slice of material 34 may be removed using the FIB to expose a new face for imaging by the SEM. As the FIB performs the milling operation ablated material 36 from the area being milled and gallium from the ion beam may build up in front of and may be redeposited back onto face 30 altering or obscuring face 20. The redeposited material in both FIGS. 2 and 3 leads to unusable data or failed automated cycles in which one poor quality slice or image can invalidate an entire run.
Software algorithms for 3D reconstruction from slice and view imaging generally assume that the surface of each slice imaged by the SEM is flat. The redepositioned material creates contrast and composition variations in which the material is interpreted as noise (e.g., decreased resolution) in the 3D representation formed from images of the exposed surfaces. For example, FIG. 4 shows a final image of a typical slice and view sample in which redeposition material 38 has built up to obscure and cast shadows 40 onto face 42 resulting in a flawed image. There is an increased demand for large volumes of flawless data because one poor quality slice or image can invalidate an entire run. Therefore, there is a need for a method that improves the acquisition of quality data from slicing process in a slice and view technique.